The Role of Ice Specimen Geometry and Impact Velocity in the Reynolds-Brook Theory of Thunderstorm Electrification

Abstract

When ice-coated spheres suspended from an insulating fiber were rotated rapidly through a stream of steadily falling natural snow crystals they acquired an electrical charge the magnitude of which increased markedly with an increase in impact velocity and degree of surface irregularity. A smooth sphere acquired a positive charge and the spheres of irregular surface structure acquired a much larger negative charge. The sign of the charging is in qualitative agreement with the temperature-gradient theory but the effects of impact velocity and surface geometry are not. Rough calculations indicate that the average charge transfer between a snow crystal and a sphere of irregular surface structure impacting at a velocity of several meters per second is several orders of magnitude greater than predicted by Mason's equations, but a comparison between these results and those emanating from the laboratory experiments of Latham and Stow indicates that the results of this field experiment are entirely explicable in terms of a temperature-gradient theory modified to accommodate these two enhancement processes. It is concluded that the experiments of Reynolds, Brook and Gourley yielded a more representative value for the average charge transfer per collision between an ice crystal and a soft-hail pellet inside a thundercloud than did the experiments of Latham and Mason, and that the Reynolds-Brook mechanism can easily generate charge inside a thunder-cloud at the minimum rate required by a tenable theory of thunderstorm electrification.